1) Field of the Invention
The present relates to a method and apparatus for vibration (e.g., sound) damping and, in particular, a constrained layer damping system with an enhanced resistance to creep, i.e., the slow lateral movement of the constraining layer relative to a surface of the source of vibration.
2) Description of Related Art
The damping of vibration of mechanical systems is of increasing importance to industry in that vibration can have a number of undesirable effects. For instance, consumers are becoming increasingly sensitive to the undesirability of sound created by vibrating systems. Also, vibration can cause electronics, mechanical joints, and fasteners to fail, and can diminish a consumer's perception of quality in a variety of products. For instance, automobile manufacturers have recognized the importance in the purchasing decision of many buyers of a solid thump sound when an automobile door is closed. Likewise, the quality of an appliance is sometimes gauged in part by the perception of the solidity of its construction.
It has become important for the manufacturers of appliances such as clothes washers and dryers, refrigerators, microwave ovens, ovens, stoves, dishwashers, etc. to provide vibration damping on the large, flat sheet material sides of the appliances so that a consumer in making his or her purchasing decision can appreciate the quality of the product by the low frequency sound generated when the side of the appliances is hit. Also, provision of such systems can be important to reduce the noise levels produced by the appliance when such sides vibrate. This is especially true today because of the increase in homes that locate such appliances on the main living floor thereof.
Sound damping systems generally operate by converting vibration energy into thermal energy. For instance, the vibration energy may be converted into thermal energy by interfacial friction, which makes it exhibit a vibration damping property. Alternatively or in addition, shear deformation may be produced within an elastic material having a small elastic modulus when it is located between a source of vibration energy and another surface or constraining layer.
For instance, Pre Finish Metals Inc. provides a product called Polycore.RTM. which consists of metal outer skins surrounding a thin, viscoelastic core material. This inner core converts the mechanical energy of vibration into heat and then dissipates the heat. This combination is purported to reduce vibration generated noise at the source.
Similarly, 3M provides products under the name "Scotchdamp.TM. vibration control systems" in which any one of a variety of adhesive layers joins a constraining layer to a source of vibrating sound. The shear modulus and sound loss factors of these products depend on frequency and temperature, as well as on other factors.
In addition to adhesives, magnetic materials may join a constraining layer to a source of vibratory sound. For instance, in U.S. Pat. No. 5,300,355, the disclosed vibration damping material includes a magnetic composite type damping material constructed by bonding an adhesive elastic sheet containing magnetic powder to a constraining plate such as a metal plate. In this system, it is reported that since not only is the damping material attracted by a magnetic force against a vibration source, it is also provided with a superficial adhesiveness to develop vibration damping properties over a wide range of temperatures.
A fundamental problem with vibration damping systems is that it is possible that the constraining layer becomes separated from the surface of the vibration source. In other words, the constraining layer can fall off during shipping or use. To measure the resistance to this form of mishap, a shock/shear test has been developed wherein an appliance, for instance, is dropped from a certain distance at a certain angle and temperature, the effect of which on the placement of a constraining layer is measured. If the adhesive properties of the adhesive are insufficient, the constraining layer can become dislodged, potentially causing mishaps within the appliance and, at a minimum, eliminating its damping effect or even perhaps increasing the perception of unwanted vibratory sound.
A further problem, however, has been perceived in that the viscoelastic materials have, as one of their properties, a limited fluidity. This property of the adhesive can permit the constraining layer to shift laterally with respect to the surface of the vibrating source, particularly when the constraining layer is vertically positioned relative to the vibration source in use. The slow movement of the constraining layer relative to the vibration source is referred to herein as "creep" and eventually can lead to catastrophic failure in the form of the constraining layer becoming loose, if the creep goes unchecked.